Buried wellbore location from surface magnetic measurements

ABSTRACT

A method for locating a buried casing stub may include a) identifying a target region, b) providing at each of a plurality of survey points in the target region a casing stub locator that includes a vector magnetometer, c) measuring the magnetic field at each of the survey points using the vector magnetometer so as to generate a plurality of magnetic field measurements, d) using the magnetic field measurements to generate a model of the magnetic field of the target region, e) fitting the model generated in step d) to a selected model of a magnetic anomaly created by the casing stub so as to generate model fit information (MFI), and f) locating the casing stub using the MFI. At each survey point, an expected Earth magnetic field can be subtracted from the measured magnetic field. A total station can measure the position and/or the azimuth of the package.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application which claims priority fromU.S. utility application Ser. No. 16/525,109, filed Jul. 29, 2019, whichis itself a non-provisional application which claims priority from U.S.provisional application No. 62/713,860, filed Aug. 2, 2018, which isincorporated by reference herein in its entirety.

TECHNICAL FIELD Field of the Disclosure

The present disclosure relates generally to wellsite operations andspecifically to locating buried casing stubs.

Background of the Disclosure

When abandoning a well, wells are often plugged below ground level.Subsequently, the wellhead is cut off and the remaining casing stub isburied. In the event that intervention of the well is necessary, such aswhere a well is improperly plugged, the casing stub must be located so aproper abandonment can take place. However, records of the well may notinclude surface location data, requiring an operator to find the buriedcasing stub before attempting the intervention operation.

Typically, buried casing stubs are located using a scalar magnetometeron the surface to map the target region where the buried casing stub islocated for magnetic anomalies. Mapping magnetometer readings to thelocation each is taken within the region may generate a “hot spot” mapthat indicates the distribution of magnetic perturbation across thetarget region. The area having the highest magnetic perturbation maygenerally indicate the location of the buried casing stub. However, anyother magnetic material in the target region such as ferrous debris,well equipment, other wells, or other magnetic materials may also bemeasured during the mapping operation by the scalar magnetometer.Additionally, scalar magnetometer readings may not indicate the depth towhich the target casing stub is buried.

SUMMARY

In some embodiments, a method for locating a buried casing stub maycomprise a) identifying a target region for the buried casing stub; b)positioning a casing stub locator at a survey point in the targetregion, the casing stub locator including a vector magnetometer; c)measuring the magnetic field at the survey point with the vectormagnetometer; d) moving the casing stub locator to a second surveypoint; e) measuring the magnetic field at the second survey point withthe vector magnetometer; f) generating a model of the magnetic field ofthe target region using the magnetic field measurements from steps c)and e); g) fitting the model generated in step f) to a selected model ofa magnetic anomaly created by the casing stub so as to generate modelfit information; and h) locating the casing stub using the model fitinformation.

The method may further comprise defining a survey grid within the targetregion; moving the casing stub locator to each point of the survey grid;and measuring the magnetic field with the vector magnetometer at eachpoint of the survey grid. The method may further comprise, for eachmeasurement location: determining an azimuth relative to true north ofthe casing stub locator; using the azimuth and position of the casingstub locator to determine expected Earth magnetic field components ofthe measured magnetic field at that measurement location; andsubtracting the Earth magnetic field components from the measuredmagnetic field at that measurement location. The method may furthercomprise positioning a total station at a location in the target regionand measuring at least one of the position of the casing stub locatorwithin the target region or the azimuth relative to true north of thecasing stub locator with the total station. The method may furthercomprise determining the location of the total station in the targetregion using a positioning device. In some embodiments, the positioningdevice may be selected from the group consisting of differential GPSunits, Global Navigation Satellite System units, or satellite navigationsystem receivers.

The azimuth relative to true north of the casing stub locator may bedetermined by visually aligning the casing stub locator to an externalreference. The casing stub locator may further comprise one or morepositioning devices adapted to locate the casing stub locating packagewithin target region and determine the azimuth of casing stub locatorand the positioning device may be a differential GPS unit. The casingstub locator may further comprise an accelerometer, and the method mayfurther comprise determining the inclination of the casing stub locatorwith the accelerometer.

The method may further comprise identifying a ferrous object inproximity to the casing stub locator by comparing the measured magneticfield to an estimated magnetic field of the casing stub; and removingthe ferrous object from the target region. The method may furthercomprise identifying a ferrous object in proximity to the casing stublocator by comparing the measured magnetic field to an estimatedmagnetic field of the casing stub; wrapping a wire around the ferrousobject; connecting the wire to a current source; and demagnetizing theferrous object by passing a current through the wire. The method mayfurther comprise the steps of e1) measuring the magnetic field with themagnetometer in proximity to the demagnetized ferrous object and e2)including the measurement made in step e1) in the model generated instep f).

In some embodiments, a casing stub locating package for use inidentifying a buried casing stub may comprise a tool body, a vectormagnetometer, the vector magnetometer adapted to detect a magneticanomaly created by the buried casing stub, an accelerometer, theaccelerometer adapted to determine the inclination of the casing stublocating package, and a positioning device, the positioning deviceadapted to determine the position of the casing stub locator package ina target region.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detaileddescription when read with the accompanying figures. It is emphasizedthat, in accordance with the standard practice in the industry, variousfeatures are not drawn to scale. In fact, the dimensions of the variousfeatures may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 depicts a target region having a buried casing stub and casingstub locator used in a surveying operation consistent with at least oneembodiment of the present disclosure.

FIG. 2 depicts a casing stub locator consistent with at least oneembodiment of the present disclosure.

FIG. 3 depicts a flow chart of a surveying operation consistent with atleast one embodiment of the present disclosure.

FIG. 4 depicts a side view of a surveying operation consistent with atleast one embodiment of the present disclosure taken between points A1and A2 of FIG. 1 .

FIG. 4A depicts a representative magnetic vector map as measured by thecasing stub locator along the surveying operation depicted in FIG. 4 .

FIG. 5 depicts a side view of the surveying operation of FIG. 4 afterremoval of ferrous debris.

FIG. 5A depicts a representative magnetic vector map as measured by thecasing stub locator along the surveying operation depicted in FIG. 5 .

FIG. 6 depicts a demagnetization apparatus used to demagnetize aferromagnetic object positioned in the target region.

FIG. 7 depicts a top view of a magnetic perturbation map as measured bythe casing stub locator representing the target region depicted in FIG.1 .

DETAILED DESCRIPTION

It is to be understood that the following disclosure provides manydifferent embodiments, or examples, for implementing different featuresof various embodiments. Specific examples of components and arrangementsare described below to simplify the present disclosure. These are, ofcourse, merely examples and are not intended to be limiting. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

FIG. 1 depicts buried casing stub 100, represented as an X located intarget region 10. Buried casing stub 100 may, for example and withoutlimitation, be the upper end of a length of casing of a previouslyabandoned wellbore. Buried casing stub 100 may therefore be formed froma ferromagnetic material such as steel and may create a local magneticanomaly in the magnetic field within target region 10 about buriedcasing stub 100. Target region 10 may, in some embodiments, beidentified as the general known location of buried casing stub 100 to besurveyed in order to locate buried casing stub 100.

In some embodiments, casing stub locator 101 may be brought to targetregion 10 and may be used in an operation to locate buried casing stub100. As discussed further herein below, casing stub locator 101 mayinclude one or more magnetometers adapted to measure the local magneticfield at the position in target region 10 at which casing stub locator101 is located. Each such magnetic field measurement is referred toherein as a survey operation. Casing stub locator 101 may berepositionable within target region 10. In some embodiments, casing stublocator 101 may be iteratively moved from position to position such thata survey is taken at each point of a grid defined within target region10, depicted in FIG. 1 as survey grid 12.

In some embodiments, a total station 50 or equivalent equipment may bepositioned at a known location in target region 10 with a line of sightto casing stub locator 101. Total station 50 may be used, for exampleand without limitation, to define survey grid 12 and to position casingstub locator 101 within target region 10. In some embodiments, totalstation 50 may include one or more positioning devices including, forexample and without limitation, a differential GPS unit, GlobalNavigation Satellite System unit, or satellite navigation systemreceiver to determine the precise location of total station 50 withintarget region 10. In some embodiments, as discussed further below, totalstation 50 may be used to measure the azimuth 17 of the long axis ofcasing stub locator 101 relative to true north before a survey operationis undertaken. In some embodiments, for example, total station 50 mayinclude a laser-reflector device to locate casing stub locator 101 thatuses one or more laser beams 51 to determine the relative distancebetween total station 50 and selected portions of casing stub locator101, from which the azimuth 17 of casing stub locator 101 can becalculated.

In some cases, one or more magnetic or ferromagnetic objects may belocated within target region 10. For illustration's sake, FIG. 1 depictsa movable ferrous object 15 and an unmovable ferrous object 20 locatedwithin target region 10. As an example, movable ferrous objects 15 mayinclude, for example and without limitation, metal waste such as buriedtrash or garbage. Unmovable ferrous objects 20 may include, for exampleand without limitation, metal culverts, existing wellheads or wells,fences, or other such structures. Ferrous objects may cause anomalousmagnetic field perturbations that may, for example and withoutlimitation, interfere with the ability of casing stub locator 101 tolocate buried casing stub 100 when a survey is taken in proximity to theferrous objects.

In some embodiments, as depicted in FIG. 2 , casing stub locator 101 mayinclude tool body 103 and stands 105. Tool body 103 may house componentsof casing stub locator 101. Stands 105 may, for example and withoutlimitation, support tool body 103 of casing stub locator 101 as it isused in survey operations. Casing stub locator 101 may include a vectormagnetometer 107. Vector magnetometer 107 may measure the magnitude anddirection of the magnetic field passing through casing stub locator 101.In some embodiments, vector magnetometer 107 may be a three-axismagnetometer. In some embodiments, casing stub locator 101 may alsoinclude one or more accelerometers 109. Accelerometers 109 may detectlocal acceleration due to gravity and may be used, for example andwithout limitation, to directly measure the inclination of casing stublocator 101 relative to a horizontal plane. In some embodiments,accelerometers 109 may be single or multi-axis accelerometers. In someembodiments, accelerometers 109 may include one or more three-axisaccelerometers.

In some embodiments, casing stub locator 101 may include one or morepositioning devices 111 positioned to accurately locate casing stublocator 101 in target region 10. In some embodiments, positioningdevices 111 may be positioned at each end of tool body 103 such that theazimuth 17 of casing stub locator 101 can be determined. In someembodiments, positioning devices 111 may each include, for example andwithout limitation, one or more differential GPS units, GlobalNavigation Satellite System units, or satellite navigation systemreceivers. In such an embodiment, the azimuth 17 and position of casingstub locator 101 may be determined without need for other tools.Additionally or alternatively, positioning devices 111 may includetargets or reflectors for use with a laser-reflector device on totalstation 50 as described herein above.

In some embodiments, casing stub locator 101 may be ameasurement-while-drilling (MWD) tool supported by stands 105.

In operation, with reference to FIG. 3 , when an intervention into awellbore associated with buried casing stub 100 is desired, targetregion 10 may first be identified (200). Target region 10 may beidentified using, for example and without limitation, historicaldrilling data associated with the wellbore associated with buried casingstub 100. Survey grid 12 of target region 10 may then be defined (210).In some embodiments, survey grid 12 may be defined using total station50. In some such embodiments, total station 50 may be located withintarget region 10 (211) using, for example and without limitation, adifferential GPS unit, Global Navigation Satellite System unit, orsatellite navigation system receiver.

Casing stub locator 101 may then be positioned within target region 10at a first survey point on survey grid 12 (220). In some embodiments,the location of casing stub locator 101 may be determined using totalstation 50. In some such embodiments, the azimuth 17 of casing stublocator 101 may be determined using total station 50 (221). In someembodiments, the inclination of casing stub locator 101 may be measuredusing accelerometers 109 (222). Casing stub locator 101 may then take asurvey by measuring the local magnetic field at the survey point usingvector magnetometer 107 (230). In some embodiments, casing stub locator101 may then be moved to a different survey point on survey grid 12(240). These operations may be repeated until a desired number ofsurveys are taken corresponding to survey grid 12. It will be understoodthat, while FIG. 4 depicts survey points arranged in a line for purposesof illustration, the survey points do not need to be arranged in a lineand may be arrayed in a grid or any other desired configuration. It willfurther be understood that, rather than moving equipment to each desiredsurvey point in sequence, surveys at two or more survey points may betaken simultaneously using multiple surveying devices, or surveys may betaken intermittently or continuously as the surveying device(s) is (are)moved continuously.

In some embodiments, during the surveying process, the magnetometer datacollected by casing stub locator 101 may be affected by identify ferrousobjects (250) other than buried casing stub 100. For example, asdepicted in FIG. 4 , a ferrous object, depicted as movable ferrousobject 15, may be positioned on or under the surface in a position suchthat the magnetic perturbations B_(d) of movable ferrous object 15 mayinterfere with the detection of the magnetic anomaly B created by buriedcasing stub 100. Similarly, in some embodiments, the magnetic anomalycreated by buried casing stub 100 may propagate as if buried casings tub100 is a magnetic monopole or magnetic dipole as shown in FIG. 4 . Byreviewing the magnetometer data collected by casing stub locator 101collected at multiple locations with the survey grid 12 and comparingeach set of collected magnetometer data to the expected model, theperturbations in the magnetic field attributable to the movable ferrousobject 15 may be identified. For example, as depicted in FIG. 4A, thevector magnetometer readings 13 a-f (shown as 2-dimensional vectors)taken at survey points 12 a-f (shown in FIG. 4 ) do not correspond withthe expected vector magnetometer readings for a magnetic dipole.

In such an instance, movable ferrous object 15 may be identified and, ifpossible, removed from target region 10 (251) as shown in FIG. 5 . Inembodiments in which movable ferrous object 15 can be removed,subsequent surveys taken at survey points 12 a-f may result inmagnetometer readings 13 a-f collected by casing stub locator 101 thatcorrespond more closely with those expected of a magnetic dipole asshown in FIG. 5A.

In the case that the interference is caused by an object that cannot bemoved, such as, for example and without limitation, where the object isunmovable ferrous object 20, the object may be demagnetized (252). Insuch a demagnetizing operation, as shown in FIG. 6 , a wire 61 may bewrapped a number of times about unmovable ferrous object 20, heredepicted as a steel wellhead. Current may be supplied to wire 61 bypower source 63. In some embodiments, the current may be supplied atvarious frequencies and current levels at predetermined intervals forpredetermined periods of time such that, without being bound to theory,the magnetic domains of the ferrous material are rearranged, resultingin a net lower magnetic field of unmovable ferrous object 20.

The magnetic field from unmovable ferrous object 20 can be mapped withthe magnetometer/accelerometer package and referenced with the totalstation. This allows the magnetic field associated with unmovableferrous object 20 to be characterized in 3D space. The magnetic fieldfrom unmovable ferrous objects 20 may then be mapped (253) usingadditional surveys taken by casing stub locator 101 such that themagnetic field of unmovable ferrous objects 20 may be characterized andtaken into consideration when locating buried casing stub 100. In thecase that additional ferrous objects are identified, such movableferrous objects 15 or unmovable ferrous objects 20 may each in turn behandled as described above.

The grid collection of data from the 3 axis magnetometer can then berepeated over the expected buried casing stub of interest. The grid canchange in latitude and departure and be repeated at different elevationsto develop a “cube” of data.

During or after this surveying process, the magnetometer data collectedby casing stub locator 101 may be used to generate a model of themagnetic field of target region 10 (260). The model may include theexpected magnetic anomaly created by buried casing stub 100 as well as,for example and without limitation, the Earth magnetic field, crustalanomalies, and other identified magnetic sources as discussed above. Insome embodiments, additional processing may be undertaken on themagnetic field data (270).

For example, in some embodiments, the knowledge of the position ofcasing stub locator 101 at each survey point as well as the inclinationand azimuth thereof may allow the expected Earth magnetic fieldcomponents of the measured magnetic field to be determined based on theexpected magnetic field at the position, inclination, and azimuth ofcasing stub locator 101, and therefore may be subtracted from themagnetic field data. The model of the magnetic anomaly—depictedtwo-dimensionally from the top in FIG. 7 as model 301—created by buriedcasing stub 100 may then be generated and may be used to locate buriedcasing stub 100 (280). In some such embodiments, the model of themagnetic field of target region 10 may be fit to an expected model ofthe magnetic anomaly created by buried casing stub 100, such as amagnetic monopole or magnetic dipole. In some embodiments, becausecasing stub locator 101 uses vector magnetometer 107, both the positionand the depth of buried casing stub 100 may be determined.

In some embodiments, a model of buried casing stub 100 may be generatedand fit using scalar magnetometer readings.

In some embodiments, buried casing stub 100 may be located without theuse of total station 50. In such an embodiment, a fit based onhorizontal and vertical magnetometer readings alone may be used toidentify a difference between the observed magnetic fields and theexpected background reference model. In other such embodiments, casingstub locator 101 may be aligned visually to an external reference toprovide a full three-dimensional model, though error may be introduceddue to inaccuracy of the positioning of casing stub locator 101.

The foregoing outlines features of several embodiments so that a personof ordinary skill in the art may better understand the aspects of thepresent disclosure. Such features may be replaced by any one of numerousequivalent alternatives, only some of which are disclosed herein. One ofordinary skill in the art should appreciate that they may readily usethe present disclosure as a basis for designing or modifying otherprocesses and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein. Oneof ordinary skill in the art should also realize that such equivalentconstructions do not depart from the scope of the present disclosure andthat they may make various changes, substitutions, and alterationsherein without departing from the spirit and scope of the presentdisclosure. Likewise, unless explicitly so indicated, the sequentialrecitation of steps in the claims that follow is not intended to be arequirement that the steps be performed sequentially.

The invention claimed is:
 1. A method for locating a buried casing stubcomprising: a) identifying a target region for the buried casing stub;b) providing a casing stub locator at each of a plurality of surfacesurvey points in the target region, the casing stub locator including avector magnetometer, the casing sub locator positioned at the surface ofthe earth; c) providing a total station in the target region, whereinthe total station has a line of sight to the casing stub locator,determining the location of the total station, and using the totalstation to measure at least one of the position of the casing stublocator within the target region or the azimuth relative to true northof the casing stub locator; d) using the vector magnetometer to measurethe magnitude and direction of the magnetic field at each of theplurality of surface survey points so as to generate a plurality ofmagnetic field measurements; e) using the plurality of magnetic fieldmeasurements to generate a model of the magnetic field of the targetregion; f) fitting the model generated in step e) to an expected modelof a magnetic anomaly created by the casing stub so as to generate modelfit information; and g) determining the position and the depth of thecasing stub using the model fit information.
 2. The method of claim 1,further comprising: defining a survey grid within the target region;moving a casing stub locator to each point of the survey grid; andmeasuring the magnetic field with the vector magnetometer at each pointof the survey grid.
 3. The method of claim 1, further comprising:determining the location of the total station in the target region usinga positioning device selected from the group consisting of differentialGPS units, Global Navigation Satellite System units, and satellitenavigation system receivers.
 4. The method of claim 1, wherein theazimuth relative to true north of the casing stub locator is determinedby visually aligning the casing stub locator to an external reference.5. The method of claim 1, wherein the casing stub locator furthercomprises one or more positioning devices adapted to locate a casingstub locating package within the target region and determine the azimuthof casing stub locator.
 6. The method of claim 5 wherein the positioningdevice is selected from the group consisting of differential GPS units,Global Navigation Satellite System units, and satellite navigationsystem receivers.
 7. The method of claim 1 wherein the casing stublocator further comprises an accelerometer, and the method furthercomprises using the accelerometer to determine the inclination of thecasing stub locator.
 8. The method of claim 1, further comprising:identifying a ferrous object in detectible proximity to the casing stublocator by comparing the measured magnetic field to an estimatedmagnetic field of the casing stub; and removing the ferrous object fromthe target region.
 9. The method of claim 1, further comprising:identifying a ferrous object within detectible proximity of the casingstub locator by comparing the measured magnetic field to an estimatedmagnetic field of the casing stub; wrapping a wire around the ferrousobject; connecting the wire to a current source; and demagnetizing theferrous object by passing a current through the wire.
 10. The method ofclaim 9, further comprising: measuring the magnetic field with themagnetometer in proximity to the demagnetized ferrous object; andincluding the measurement made in proximity to the demagnetized ferrousobject in the model generated in step e).
 11. The method of claim 1,further comprising, for each survey point: determining an azimuthrelative to true north of the casing stub locator; using the azimuth andposition of the casing stub locator to determine expected Earth magneticfield components of the measured magnetic field at that survey point;and subtracting the Earth magnetic field components from the measuredmagnetic field at that survey point.